There has been a growing interest in the manufacture and use of microfluidic systems for the acquisition of chemical and biochemical information. Techniques commonly associated with the semiconductor electronics industry, such as photolithography, wet chemical etching, etc., are being used in the fabrication of these microfluidic systems. The term,"microfluidic", refers to a system or device having channels and chambers which are generally fabricated at the micron or submicron scale, e.g., having at least one cross-sectional dimension in the range of from about 0.1 .mu.m to about 500 .mu.m. Early discussions of the use of planar chip technology for the fabrication of microfluidic systems are provided in Manz et al., Trends in Anal. Chem. (1990) 10(5):144-149 and Manz et al., Avd. in Chromatog. (1993) 33:1-66, which describe the fabrication of such fluidic devices and particularly microcapillary devices, in silicon and glass substrates.
Applications of microfluidic systems are myriad. For example, International Patent Appln. WO 96/04547, published Feb. 15, 1996, describes the use of microfluidic systems for capillary electrophoresis, liquid chromotography, flow injection analysis, and chemical reaction and synthesis. A related patent application, U.S. application Ser. No. 08/671,987, entitled "HIGH THROUGHPUT SCREENING ASSAY SYSTEMS IN MICROSCALE FLUIDIC DEVICES", filed Jun. 28, 1996 by J. Wallace Parce et al. and assigned to the present assignee, discloses wide ranging applications of microfluidic systems in rapidly assaying compounds for their effects on various chemical, and preferably, biochemical systems. The phrase,"biochemical system" generally refers to a chemical interaction that involves molecules of the type generally found within living organisms. Such interactions include the full range of catabolic and anabolic reactions which occur in living systems including enzymatic, binding, signaling and other reactions. Biochemical systems of particular interest include, e.g., receptor-ligand interactions, enzyme-substrate interactions, cellular signaling pathways, transport reactions involving model barrier systems (e.g., cells or membrane fractions) for bioavailability screening, and a variety of other general systems.
Many methods have been described for the transport and direction of fluids, e.g., samples, analytes, buffers and reagents, within these microfluidic systems or devices. One method moves fluids within microfabricated devices by mechanical micropumps and valves within the device. See, Published U.K. Patent Application No. 2 248 891 (Oct. 18, 1990), Published European Patent Application No. 568 902 (May 2, 1992), U.S. Pat. Nos. 5,271,724 (Aug. 21, 1991) and 5,277,556 (Jul, 3, 1991). See also, U.S. Pat. No. 5,171,132 (Dec. 21, 1990) to Miyazaki et al. Another method uses acoustic energy to move fluid samples within devices by the effects of acoustic streaming. See, Published PCT Application No. 94/05414 to Northrup and White. A straightforward method applies external pressure to move fluids within the device. See, e.g., the discussion in U.S. Pat. No. 5,304,487 to Wilding et al.
Still another method uses electric fields, and the resulting electrokinetic forces, to move fluid materials through the channels of the microfluidic system. See, e.g., Published European Patent Application No. 376 611 (Dec. 30, 1988) to Kovacs, Harrison et al., Anal. Chem. (1992) 64:1926-1932 and Manz et al. J. Chromatog. (1992) 593:253-258, U.S. Pat. No. 5,126,022 to Soane. Electrokinetic forces have the advantages of direct control, fast response and simplicity. However, there are still some disadvantages with this method of operating a microfluidic system.
Present devices use a network of channels in a substrate of electrically insulating material. The channels connect a number of fluid reservoirs in contact with high voltage electrodes. To move fluid materials through the network of channels, specific voltages are simultaneously applied to the various electrodes. The determination of the voltage values for each electrode in a system becomes complex as one attempts to control the material flow in one channel without affecting the flow in another channel. For example, in a relatively simple arrangement of four channels intersecting in a cross with reservoirs and electrodes at the ends of the channels, an independent increase of fluid flow between two reservoirs is not merely a matter of increasing the voltage differences at the two reservoirs. The voltages at the other two reservoirs must also be adjusted if their original flow and direction are to be maintained. Furthermore, as the number of channels, intersections, and reservoirs are increased, the control of fluid through the channels become more and more complex.
Also, the voltages applied to the electrodes in the device can be high, i.e., up to a level supportive of thousands of volts/cm. Regulated high voltage supplies are expensive, bulky and are often imprecise and a high voltage supply is required for each electrode. Thus the cost of a microfluidic system of any complexity may become prohibitive.
The present invention solves or substantially mitigates these problems of electrokinetic transport in a microfluidic system which uses another electrical parameter, rather than voltage, to simplify the control of material flow through the channels of the system. A high throughput microfluidic system having direct, fast and straightforward control over the movement of materials through the channels of the microfluidic system with a wide range of applications, such as in the fields of chemistry, biochemistry, biotechnology and molecular biology and numerous other fields, is possible.